There is a microphone as an acoustoelectric transducer. In general, in order to provide sharp directivity for sensitivity in an incident direction of a sound wave vertical to a vibrating board of the microphone, it is necessary to configure a microphone apparatus so as to have the sound wave incident not only on a front portion but also on a back portion of the vibrating board.
As for a dynamic microphone broadly used in the past, it has a configuration wherein a coil is mounted on the vibrating board in order to detect the sound wave from the vibrating board, and so the coil and so on resist sound pressure entering from the back so that the vibrating board cannot always be vibrated as on the front. It was difficult, however, to provide the configuration wherein the front portion and the back portion of the vibrating board are completely opened so as to render the sound wave incident from both the front portion and the back portion.
In addition, as for a condenser microphone, it has the configuration wherein, as it detects the sound wave by detecting change of capacity due to vibration of the vibrating board, the back cannot be structurally opened to render the sound wave incident from the backside. Accordingly, it is ideal that the acoustoelectric transducer such as the microphone has nothing on its back as on its front.
Moreover, an optical microphone apparatus using an optical device is known as one of the microphones.
For instance, Japanese Patent Application Laid-Open No. 8-297011 discloses an optical fiber sensor using a pair of optical fibers and having a configuration wherein light is irradiated to a vibration medium from one optical fiber connected to a light source and the light is detected by the other optical fiber, indicating that it is applicable to a microphone.
An optical microphone device used for the optical microphone apparatus is comprised of the vibrating board for vibrating due to sound pressure, the light-emitting device for irradiating a light beam on this vibrating board, and the light-receiving element for receiving reflected light from the vibrating board and outputting a signal corresponding to vibration displacement of the vibrating board.
Thereby it is possible to detect the vibration displacement of the vibrating board caused by the fact that the sound wave hits the vibrating board without touching this vibrating board and to convert the detected vibration displacement to an electric signal, so that it is no longer necessary to place a vibration detecting system on the vibrating board, weight of the vibrating portion can be rendered lighter, and feeble variation of the sound wave can be sufficiently followed.
A first objective of the present invention is, for the purpose of solving the above-mentioned first problem, to provide the acoustoelectric transducer having the directivity, as its directional characteristic, only in the vertical direction to the vibrating board.
In addition, as for the microphone in the past, the apparatus is configured by using a single optical microphone device so that one vibrating board covers frequency characteristics ranging from low to high frequencies.
Such a microphone characteristic is generally called a monotone characteristic, where frequency coverage is actually almost limited to 50 Hz to 20 KHz as shown in FIG. 11.
Thus, as the optical microphone apparatus in the past used a single optical microphone device using a single vibrating board, it is difficult to control the low to high frequencies with the single vibrating board so as to render the sensitivity (amplitude) thereof flat. In general, the sensitivity in a low frequency band is relatively enhanced by increasing thickness of the vibrating board, and the sensitivity in a high frequency band is enhanced by decreasing the thickness thereof.
Accordingly, it is difficult, due to such a physical property of the vibrating board, to implement the optical microphone apparatus of which frequency characteristic of the sensitivity (amplitude) is flat over a wide frequency band.
A second objective of the present invention is, for the purpose of solving such a second problem in the past, to provide the acoustoelectric transducer like the optical microphone apparatus of which sensitivity (amplitude) characteristic is flat over a wide frequency band.
Furthermore, in case of configuring the optical microphone apparatus of the wide frequency band by arranging a plurality of the past optical microphone devices, there is a fault that the vibrating board cannot be rendered close or the shape thereof becomes larger. For that reason, it is difficult to implement a small and wide-band directional microphone apparatus.
Moreover, as the size of the vibrating board of the microphone apparatus is fixed, it is difficult to have settings with featured frequency characteristics and to implement the microphone apparatus which is efficient in the wide frequency band.
A third objective of the present invention is, for the purpose of solving the above-mentioned third problem, to provide the directional acoustoelectric transducer which is small and has the wide-band frequency characteristic.